I do know that real authentic double decal Waffen-SS helmets (that have not had decals ADDED to them post war by people trying to increase their value) are a rarity.
Indeed, I collect original helmets as well, but have no ss or decalled helmets. Even they are getting more and more expensive.
They eliminated the national shield on heer,luftwaffe,kriegsmarine helmets and the party shield on the ss in 1940 because they made great visible targets. Of course as previously mentioned many troops didn't remove them and many earlier helmets with decals were re-issued, but after 43 they came from the factory without decals.